Supplementation of adult Atlantic salmon (Salmo salar) restores biofilm biomass and benthic metabolism

Restoration of adult Atlantic salmon ( Salmo salar ) populations can restore key ecological functions in nutrient-limited freshwater ecosystems. This study assessed the effects of marine-derived nutrients (MDNs) from spawning salmon on biofilm biomass and benthic metabolism in two adjacent watersheds, the Upper Salmon River (USR) and Point Wolfe River (PWR), located in and around Fundy National Park, New Brunswick, Canada. Using a Before-After-Control-Impact (BACI) design, between 2015–2017 we monitored changes in biofilm growth, metabolism, and stable isotope signatures across upstream (reference) and downstream (treatment) sites in both rivers. Significant increases in biofilm biomass (up to 0.099 μg chlorophyll m −2 ) and net daily metabolism (up to 13.1 mg O₂ m⁻² d⁻¹) were observed at treatment sites in the USR where adult salmon were released. In contrast, no such increases occurred in reference sites, or the non-supplemented PWR. Stable isotope analysis confirmed MDN assimilation in biofilm and linked enriched δ¹ ³C and δ¹ ⁵N values to the presence of adult salmon. A mass balance nutrient model demonstrated that adult salmon, unlike juvenile stocking, deliver net nutrient gains to streams. This study provides the first empirical evidence for functional ecosystem recovery in Atlantic salmon rivers via adult supplementation, emphasizing the role of salmon as a keystone species and supporting their use in both demographic and ecological restoration efforts in the endangered inner Bay of Fundy population range. • Biofilm accrual increased significantly in areas receiving marine-derived nutrients. • Net daily metabolism also linked to spawner abundance and marine-derived nutrients. • Increased levels of biomass and productivity were maintained into the winter months. • Stable isotope analysis confirm increases directly linked to marine-derived nutrients. • Enhancement through mature adults contributes to recovery and ecological restoration.